In petroleum products, n-alkanes have molecular diameter smaller than 5A, while isoalkanes, cycloalkanes, and aromatics have molecular diameter greater than 5A. According to the adsorption characteristic of molecular sieves, n-alkanes can be separated and absorbed from petroleum products with 5A molecular sieves as the adsorbent. Presently, the separation technique for separating n-alkanes from petroleum products mainly used by abroad industrial installations is the Molex molecular sieve dewaxing process developed by UOP company (USA), and the molecular sieve dewaxing adsorbents used mainly are ADS-12, ADS-14, and ADS-34 series products from UOP Company.
The NWA-IIB molecular sieve adsorbent from SINOPEC Catalyst (Nanjing) is developed with independently developed techniques by spraying silica beads first, and then performing in-situ crystallization and exchange to synthesize 5A bead adsorbent. That process has advantages such as free of binder, high molecular sieve content, small crystal grain size, high strength, and continuous production, etc., but has disadvantages such as long process, low yield of silica beads, oily waste water with high ammonia nitrogen content, high product cost, and unmodifiable properties, etc.
The Chinese patent document CN87105499A has disclosed a method for preparation of binder-free spherical type A molecular sieve, which prepares silica hydrogel beads from inorganic ammonium salt, inorganic acid, and sodium silicate through an oil column forming process, produces low-density loose silica beads through water washing, surfactant impregnation, drying, and calcination, mixes the silica beads with sodium metaaluminate solution, performs aging and crystallization at specific temperature so that the silica are essentially transformed into 4A molecular sieves, and then turn the 4A molecular sieves into 5A molecular sieves through calcium exchange. The process brings two major pollutants in the production process, and the pollutants are difficult to treat; especially, the use of inorganic ammonium salt results in ammonia nitrogen waste water that is difficult to treat, and the use of surfactant causes COD out-of-specification of the waste water.